How to perform the hack squats correctly and safely which will help prevent injury.

Muscles Involved in order of importance:

Quadriceps

Glutes

Hamstrings

To perform this exercise you should use place your shoulders under the shoulder pads and your back against the back rest and your feet about shoulder width apart and your toes slightly pointed out.
Slowly lower the weight down until your legs are a little bit under 90 degrees.
Then push with your legs back up until you are at the starting posistion.

Safety: Doing this exercise should be done with a spotter if you think the weight will be a bit too much, I also have a spotter when doing this exercise just to be on the safe side.

You can try different foot positions when doing this exercise to put emphasize on your legs